David Lewis (1932- ) has written several books on Christian themes, including Praying for Special Days and Praying for Family.  He also wrote several children’s books for Christian publisher Scripture Union, amongst which is The Secret in the Hills.  

Finding the book:  very easy to find, and usually cheap.

Sources and links:

The British Library catalogue

David Lewis

The Secret in the Hills

Leopard Books, Scripture Union, London 1978, pb, 95 pp.

Reprinted 1981, 1983

Andy’s secret leads him into danger in the Welsh hills, and then he discovers a far greater secret.

Bibliography - pony books only